/**
*      author: Y_Yao
*      created: 2024-10-21 20:15:01
**/

#include<bits/stdc++.h>

#define int LL
using namespace std;
using LL = long long;
const LL LLINF = 1e18;
const int INF = 0x3f3f3f3f;
const int N = 5e5+ 7;
int a[N];
int L,n,m;

bool check(int mid)
{
    int prev = 0;
    int ret = 0;
    for(int i = 0;i <= n;i++)
    {
        if(a[i] - prev >= mid)
        {
            ret++;
            prev = a[i];
        }
    }
    if(L - a[n] >= mid) ret++;
    if(ret >= n - m + 1) return true;
    else return false;
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    cin >> L >> n >> m;
    for(int i = 1;i <= n;i++)
    {
        cin >> a[i];
    }

    int l = 1,r = L;
    while(l < r)
    {
        int mid = (l + r + 1) >> 1;
        if(check(mid)) l = mid;
        else r = mid - 1;
    }
    cout << l << endl;
    return 0;
}